CVE-2023-38686

CRITICALNVD 9.39.3—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.3 under NVD CVSS v3. EPSS exploit probability: 0.2%, top 86% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
Elevated
9.3
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 0%CVSS: 9.3Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

Sydent is an identity server for the Matrix communications protocol. Prior to version 2.5.6, if configured to send emails using TLS, Sydent does not verify SMTP servers' certificates. This makes Sydent's emails vulnerable to interception via a man-in-the-middle (MITM) attack. Attackers with privileged access to the network can intercept room invitations and address confirmation emails. This is patched in Sydent 2.5.6. When patching, make sure that Sydent trusts the certificate of the server it is connecting to. This should happen automatically when using properly issued certificates. Those who use self-signed certificates should make sure to copy their Certification Authority certificate, or their self signed certificate if using only one, to the trust store of your operating system. As a workaround, one can ensure Sydent's emails fail to send by setting the configured SMTP server to a loopback or non-routable address under one's control which does not have a listening SMTP server.

CVSS v3
9.3
EG Score
9.3(medium)
EG Risk
58(Track*)
EG Risk 58/100SSVC: Track*

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity93% × 45%
Exploitation40% × 40%
Automatability0% × 15%
Action: Watch closely — could escalate to Attend.
EPSS
13.8%
KEV
Not listed

Published

August 4, 2023

Last Modified

November 21, 2024

Advisory Details (7)

Auto-updated Jul 17, 2026
⚠️ Active exploitation confirmed. Patch available. Sources: github_commit, github_pr, github_release, github.
github_pr Patch Available

Explicitly create an SSL context when emailing

Patch available: matrix-org/sydent v2.5.6 (PR #574 merged 2023-07-31)

https://github.com/matrix-org/sydent/pull/574
github_commit Patch Available

commit 1cd748307c6b (matrix-org/sydent)

Patch available: matrix-org/sydent v2.5.6 (contains commit 1cd748307c6b)

https://github.com/matrix-org/sydent/commit/1cd748307c6b168b66154e6c4db715d4b9551261
generic

ssl — TLS/SSL wrapper for socket objects — Python 3.14.6 documentation

https://docs.python.org/3/library/ssl.html?highlight=ssl#security-considerations
generic

PEP 476 – Enabling certificate verification by default for stdlib http clients | peps.python.org

https://peps.python.org/pep-0476/
generic🔴 Active Exploitation

Enable TLS certificate validation by default for SMTP/IMAP/FTP/POP/NNTP protocols · Issue #91826 · python/cpython · GitHub

https://github.com/python/cpython/issues/91826
github Patch Available

Sydent does not verify email server certificates · Advisory · matrix-org/sydent · GitHub

https://github.com/matrix-org/sydent/security/advisories/GHSA-p6hw-wm59-3g5g
github_release Patch Available

v2.5.6

Patch available: matrix-org/sydent v2.5.6

https://github.com/matrix-org/sydent/releases/tag/v2.5.6

Affected Packages

(1 across 1 ecosystem)
PyPI(1)
PackageVulnerable rangeFixed inDependents
matrix-sydent2.0.0 ... 2.5.5 (18 versions)2.5.6—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 13× in last 7d / 53× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 112 total refreshes for this CVE.

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Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2023-38686?
CVE-2023-38686 is a critical vulnerability published on August 4, 2023. Sydent is an identity server for the Matrix communications protocol. Prior to version 2.5.6, if configured to send emails using TLS, Sydent does not verify SMTP servers' certificates. This makes Sydent's emails vulnerable to interception via a man-in-the-middle (MITM) attack. Attackers with…
When was CVE-2023-38686 disclosed?
CVE-2023-38686 was first published in the National Vulnerability Database on August 4, 2023, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2023-38686 actively exploited?
CVE-2023-38686 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 13.8% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2023-38686?
CVE-2023-38686 has a CVSS v3 base score of 9.3 (NVD).
How do I remediate CVE-2023-38686?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2023-38686, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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